The present invention relates to a sheet-like light source device. More particularly, the present invention relates to a back light device of a perpendicular type to be used as a back light of a liquid crystal display.
A non-light emission display device such as a liquid crystal display is provided with a sheet-like light source device referred to as a back light to irradiate uniform light on an image display element from the back. For the back light, a back light of a side-light type has been often used with a reduction in the thickness of the liquid crystal display.
In the side-light type, however, an increase in the size of the screen and the precision of a recent liquid crystal display is limited. For this reason, a back light having a higher luminance has been required than that of the back light of the side-light type which is a current mainstream. For the request, recently, a back light of a perpendicular type shown in FIGS. 9 and 10 has often been manufactured. FIG. 9 is an exploded perspective view and FIG. 10 is a sectional view.
More specifically, the back light of the perpendicular type has a plurality of light sources 21 (for which a fluorescent lamp is mainly used) provided in a space under a display device 25 and serves to guide light of the light source 21 to the display device 25. In that case, a reflecting plate 22 is provided on the opposite side of the display device 25 with respect to the light source 21 in order to efficiently guide light of the light source 21 to the display device 25 side. In many cases, it is hard to control the direction of light emitted from a lamp. Therefore, a white reflecting plate 22 for diffusing and reflecting light is selected. Moreover, a diffusing plate 23 is provided between the light source 21 and the display device 25 in order to make the light emitted from the light source 21 uniform. In some cases, furthermore, optical sheets (not shown) such as a prism sheet for collecting light in a desirable direction is provided between the diffusing plate 23 and the display device 25.
In a conventional back light device, the light (direct light) directly emitted from the light source 21 to the light emitting face side is directly incident on the diffusing plate 23, and light emitted to the anti-light emitting face side is reflected by the reflecting plate 22 and is then incident on the diffusing plate 23 (reflected light). As a result, a luminance is high on a display surface just above the light source 21 and a display unevenness is recognized. For this reason, a light uniform processing is carried out, that is, the diffusing plate 23 is subjected to printing for shielding light. Thus, uniform light emission can be obtained.
In the conventional back light device of the perpendicular type, a dimension in a vertical direction is less restricted than that of the back light device of the side-light type and the degree of a display unevenness is also lessened. The above technique can be employed because uniform light emission can be obtained by only the printing on the diffusing plate 23. In the near future, however, it is apparent that a back light device having a small thickness, a narrow frame and a high efficiency is required for the back light device of the perpendicular type in the same manner as the back light device of the side-light type. In the thin back light of the perpendicular type, a ratio of direct light to reflected light is different from that of the conventional back light and the direct light is more increased than the reflected light. For this reason, the light shield printing on the diffusing plate 23 cannot fully make the light uniform. Thus, there has been a problem that the display unevenness is more remarkable. Moreover, when the intense direct light is shielded through the light shield printing subjected to the diffusing plate 23 in the same manner as that in the conventional art, the utilization efficiency of the light of the back light is reduced so that the efficiency of the back light is deteriorated.
Japanese Unexamined Patent Publication No. 275525/1992 or the like has disclosed a technique for making light uniform without using the light shielding means. In a liquid crystal display having a back light described in the publication, a reflecting plate is protruded upward like a mountain in the middle position of two adjacent light sources, and a convex portion of the reflecting plate serves as a mirror surface and a concave portion thereof serves as a scattering surface. Accordingly, it is necessary to carry out a working and an assembly with high precision in order to actually apply these techniques or a plurality of expensive members should be additionally used, which is contrary to the fabrication of a back light at a low cost. Therefore, it is hard to fabricate an inexpensive plane light source through these techniques.
The present invention has been made to eliminate the above-mentioned drawbacks, it is an object thereof to provide a sheet-like light source device which is inexpensive and thin and has a uniform luminance distribution while minimizing a reduction in the utilization efficiency of light.
In accordance with the present invention, there is provided a sheet-like light source device comprising a plurality of bar-shaped light sources, a diffusing plate for diffusing light emitted from the light sources, and a reflecting plate capable of mirror reflecting the light emitted from the light sources to the diffusing plate side, the reflecting plate being provided in a position opposite to the diffusing plate with respect to the light sources,
wherein the reflecting plate has at least one convex portion formed in parallel with the light sources in a space between the two adjacent light sources.
It is preferable that the convex portion takes a sectional shape having three or more straight portions.
It is preferable that a straight portion forming at least a base bottom part of the convex portion is set to have such an angle as to reflect the light emitted from the light source onto a part almost just above the light source, and
a straight portion provided on the base bottom part of the convex portion is set to have such an angle as to reflect the light emitted from the light source onto a portion between the two adjacent light sources.
It is preferable that an inclination of each straight portion constituting the sectional shape of the convex portion has a relationship of:
xcex81 less than xcex82 less than  . . .  less than xcex8nxe2x80x83xe2x80x83(1)
(wherein xcex8n represents an angle formed by the straight portion and a normal of the diffusing plate and n=1 represents a straight portion forming the base bottom part of the convex portion of the reflecting plate).
It is preferable that the sectional shape of the convex portion is symmetrical with respect to a center line of the two adjacent light sources arranged in parallel.
It is preferable that a straight portion constituting the section of the convex portion which is the closest to the diffusing plate is parallel with the diffusing plate.
It is preferable that both ends of the reflecting plate in a direction of the arrangement of the light sources is constituted by a portion perpendicular to the diffusing plate and a half on one of sides of the convex portion.
It is preferable that the convex portion has a plurality of sectional shapes in an axial direction of the light source.
It is preferable that the sectional shapes of the convex portion is continuously changed in the axial direction of the light source.
It is preferable that a plurality of convex portions are provided and two adjacent convex portions have different sectional shapes.
It is preferable that a wiring of the light source is accommodated on an internal face side of the convex portion.
In accordance with present invention, there is also provided a sheet-like display device comprising a sheet-like light source device and at least a plane type display part, wherein the sheet-like source device comprises a plurality of bar-shaped light sources, a diffusing plate for diffusing light emitted from the light sources, and a reflecting plate capable of mirror reflecting the light emitted from the light sources to the diffusing plate side, the reflecting plate being provided in a position opposite to the diffusing plate with respect to the light sources, and wherein the reflecting plate has at least one convex portion formed in parallel with the light sources in a space between the two adjacent light sources.
It is preferable that the plane type display part displays an image by utilizing a birefringence of a liquid crystal.
It is preferable that a member for an electric circuit is accommodated on an internal face side of the convex portion.
More specifically, the technical means of the present invention pays attention to the fact that the light emitted from the light source toward the reflecting plate has conventionally been reflected by diffusion reflection and has not been controlled. Thus, the present invention has an object to provide a back light which is thin and inexpensive and has a high efficiency by using a reflecting surface of the reflecting plate to carry out mirror reflection, thereby controlling the light and minimizing or deleting printing to be performed over the diffusing plate. Moreover, other members for making the light uniform are not required. Therefore, it is possible to fabricate an inexpensive sheet-like light source during mass production.